Butler matrices are generally employed for microwave-frequency signals or more generally for electrical signals in the microwave frequency range. The technology conventionally used to produce a Butler matrix is waveguide technology which exhibits the drawback of significant bulkiness. Indeed, for onboard applications, a problem to be solved relates to the miniaturization of such devices since the compactness of an antennal device is a significant advantage especially when the number of antennal elements, and therefore indirectly the number of outputs of the Butler matrix, increases.
A known solution making it possible to solve the problem of bulkiness of the Butler matrices produced with waveguide technology consists in converting the electrical signal at a microwave frequency into an optical signal so as to be able to produce the Butler matrix consisting of an arrangement of couplers and delay lines implemented by a photonic integrated circuit or PIC. The wavelength of an optical signal being by nature substantially more reduced than that of an electrical signal at a microwave frequency, the compactness of the device is thus improved.
Solutions for implementing Butler matrices utilizing the technology of photonic integrated circuits are described notably in the scientific publication “Experimental Demonstration of Optical Guided-Wave Butler Matrices, “by J.T. Gallo and R. DeSalvo, IEEE Transactions on Microwave Theory and Techniques, Vol. 45, No. 8, August 1997” or in the international publication of application WO 2003/079101.
The proposed architectures are based on heterodyne optical systems which require very precise control of the optical phase and implementation of complex phase control loops.
A problem with this type of architecture relates to the precision of the phase shifters required to produce a Butler matrix. Indeed, as indicated hereinabove a Butler matrix is traditionally composed of hybrid couplers but also of phase shifters. The arrangement of these couplers and phase shifters makes it possible to produce the desired matrix transfer function, which must be unitary or at least orthogonal, so as notably to configure the phases of the output signals of the matrix. For a matrix with 8 inputs and 8 outputs, the necessary phase shifts are multiples of PI/8 or 22.5°. A phase shifter is in practice embodied by a delay line. Now, in the optical domain, the wavelength (directly related to the phase shift to be imparted) is much reduced, typically of the order of a few nanometres. It is therefore seen that a problem exists relating to the precision of the embodiment of the delay lines to implement the desired phase shifts with the required accuracy. The precision of the phase shifts is significant since it is directly related to the mutual isolation of the output pathways of the matrix. If the phase shifts are not implemented in a sufficiently precise manner, this has an impact on the transfer function of the matrix which is then no longer unitary.
A problem therefore exists in respect of improving the precision of the phase shifters of a Butler matrix implemented in PIC technology.
The present invention affords a response to the aforementioned problem by proposing a distributed feeding circuit having small bulk, since it can be embodied in PIC technology, adapted for receiving an electrical signal modulated on optical carrier.
The invention exhibits the main advantage of allowing configuration of the lengths of the delay lines on the scale of the microwave frequencies of the electrical signal, thereby greatly facilitating the implementation of the precise phase shifts such a circuit must comprise.
The distributed feeding circuit according to the invention, allows, when it is employed in an antenna beamforming array, the generation of a multiple antenna beam in directions of pointing whose angular spacing is adjustable.